| Ablation | The erosion of a solid body by a high-temperature gas stream moving with high velocity, e.g., a re-entry vehicle’s heat shield which melts or chars under the effects of air friction. |
| Abort | To cancel or cut short a mission. |
| Absolute Zero | The temperature at which all heat action ceases, −273.16°C (−459.69°F). |
| Acceleration | A change in velocity, including changes of direction and decreases as well as increases in speed. |
| Accelerometer | A device that senses changes in speed along its axis. |
| Active Sun | The Sun during times of frequent solar activity such as sunspots, flares, and associated phenomena. |
| Aerobraking | The process of decelerating by converting velocity into heat through friction with a planetary atmosphere. |
| Aeronautics | The science of building and operating vehicles for flight. |
| Albedo | Reflectivity; the ratio of reflected light to incident light. The fraction of sunlight reflected off a planet. |
| Angstrom | A unit for the measurement of wavelength equal to one hundred millionth of a centimetre. |
| Annular | Pertaining to, or having the form of a ring. |
| Anomaly | The angular distance between the position of a planet and its last perihelion, or between that of a satellite and its last perigee. |
| Artificial Gravity | Use of centrifugal force to simulate weight reaction in a condition of free-fall. |
| Asteroid | A small, usually irregularly shaped body orbiting the Sun, most often between the orbits of Mars and Jupiter. |
| Asteroid Belt | A region between Mars and Jupiter where most asteroids are found. |
| Astronaut | A person who flies in space as a crew member or passenger. |
| Astronautics | The science and technology of space flight. |
| Astronomical Unit (AU) | The mean distance of Earth from the Sun, about 149,597,870 km. |
| Astrophysics | The study of the physical and chemical nature of celestial bodies and their environments. |
| Atmospheric Pressure | The weight of air on surfaces within Earth’s atmosphere, about 14.7 PSI at sea level. |
| Attitude Control | The system that turns and maintains a spacecraft in the required direction. |
| Backscattering | Reflecting light back in the direction of the source. |
| Ballistics | The science dealing with the motion and behaviour of projectiles or missiles. |
| Battery | A device with connected cells that produce electric current by converting chemical energy into electrical energy. |
| Big Bang Theory | The theory that the universe began from a highly dense and hot state and has been expanding ever since. |
| Binary Star | Two stars revolving around a common center of gravity. |
| Celestial Sphere | An imaginary sphere surrounding Earth used to locate celestial objects. |
| Centrifugal Force | A force directed away from the center of rotation. |
| Centripetal Force | A force directed toward the center of rotation. |
